The present invention relates to a down converter for RF signals, a demodulator comprising a down converter, a mobile communications device comprising such a demodulator, a method for down converting RF signals by means of one three port junction as well as to a demodulation method.
The proposed technique is particularly convenient for down converting and demodulating RF signals modulated with a modulation scheme with constant envelope such as f.e. nPSK (Phase Shift Keying).
The so-called six-port technology which has been used up to the last years for complex measurements can also be utilised for designing receivers. Receiver built on the basis of a six-port assembly act in the direct conversion manner allowing therefore a conversion from for example signals in the mm-wave range and microwave range directly to a base band frequency. The main feature of the six-port receiver thereby is the detection of the vector ratio between two incoming vector signals by means of a power sensing at different positions within the RF circuitry of the six-port topology. A direct six-port receiver is for example described in Ji Li, R. G. Bossisio and Ke Wu: xe2x80x9cA six port direct digital millimeter wave receiverxe2x80x9d, Digest of IEEE MTT Symposium, vol.3, pp 1659xe2x89xa71662, San Diego, May 1994.
From the WO99/08426 a non-coherent six-port receiver is known. This known receiver comprises a power divider to divide a modulated RF input signal in at least two branches. At least one delay line provides for a delay of the branches relatively to each other by a predetermined delay constant. A calculation circuit calculates at least three power levels based on combinations of the two branches of the input signal relatively delayed to each other. A processing means calculates the phase and the amplitude of a complex signal representing the relation between the two branches of the input signal relatively delayed to each other, on the basis of the said at least three power levels.
It is the object of the present invention to further develop the above-cited non-coherent six-port receiver technology such that the complexity of the necessary circuitry can be reduced.
This object is achieved by means of the features of the independent claims. The depending claims developed further the central idea of the present invention.
The new technology according to the present invention allows the same functionality as a non-coherent six-port receiver topology as it is set forth in the WO99/08426. However, the RF circuitry can be significantly reduced and only two power sensors are necessary. A local oscillator can be omitted. In case of simple modulation techniques like (n) PSK there is furthermore no need for a RF switch.
According to a first aspect of the present invention therefore a down converter for RF signals is provided, wherein the down converter comprises one three port junction device. The three port junction device has one input port and two output ports, wherein the output ports are respectively connected to a power sensor.
The down converter can be a non-coherent down converter without any local oscillator.
The three port junction device can comprise a power splitter dividing the signal supplied to the input terminal into two branches. Furthermore a processing means for processing one of the branches and a four port junction device for combining the two branches and for generating two output signals to be supplied to the output terminals are comprised in the three port junction device.
The processing means can comprise a delay unit.
Alternatively the processing means can comprise a frequency divider, at least one filter means and a frequency multiplier.
The dividing factor of the frequency divider is equal to the multiplying factor of the frequency multiplier.
A switch can be provided in the signal branch processed by the processing means.
According to the present invention furthermore a demodulator comprising a down converter as set forth above is provided. The signal supplied to the input port in this case is a digital modulated signal and the switch is controlled to be opened during a time duration corresponding to one half of the bit duration of the supplied digital modulated signal.
Respectively one A/D converter can be connected directly or indirectly with one of the power sensor.
According to another aspect of the present invention a demodulator comprising a down converter as set forth above is provided. At least one averaging unit is connected directly or indirectly with the output of the power sensor and the output of the averaging units is supplied together with signals corresponding to the input signal of the at least one averaging unit to at least one analog processing unit.
According to the present invention furthermore a mobile communications device comprising a demodulator as set forth above is provided.
According to a still further aspect of the present invention a method for down converting RF signals by means of one three port junction device is provided. A RF signal is supplied to one input port of the three port junction device. Two output signals are generated on the basis of the one input RF signal. The output signals are supplied at two output ports of the three port junction device respectively to a power sensor.
The step of generating two output signals on the basis of the input RF signal can comprise the step of dividing the signal supply to the input terminal to two branches. One of the branches is processed and the two branches are combined, wherein as a result of the combination of the two branches two output signals to be supplied to the output terminals and thus to the power sensors are generated.
The step of processing one of the branches can comprise the step of delaying the signal of the corresponding branch by a time duration corresponding to the time duration of at least one modulation bit of the input RF signal.
The step of processing one of the branches alternatively can comprise the steps of frequency dividing, filtering and frequency multiplying the signal of the processed branch.
The dividing factor of the frequency dividing step is equal to the multiplying factor of the frequency multiplying step.
The processed signal branch can be optionally switched on and off.
According to a still further aspect of the present invention a demodulating method is provided comprising the steps of the down-converting method as set forth above. In this case the signal supply to the input port of the three port junction device is a digital modulated signal and the switching is controlled to be opened during a time duration corresponding to one half of the bit duration of the supplied signal.
The demodulating method can comprise the step of A/D converting at least one signal based on output signals of the power sensors.
The demodulating method can comprise a down-converting method as set forth above and furthermore is still a step of averaging the output of at least one of the power sensors and analog processing the output of the averaging step and signals corresponding to the input signal of the averaging step.
More than one demodulation state can be generated by the analog processing step, such that additional information is obtained indicating the quality of the demodulation. This (soft) information can be used in a decoding step.
The demodulating method can comprise the steps of A/D converting the output of the analog processing step with more than two bits, digitally processing the A/D converted signals and supplying the digitally processed signals to a soft decision unit.